With modern methods of materials processing, structures can now be fabricated on the nanometer-length scale. As is well-known in the art, techniques developed in the semiconductor industry (for example, electron beam lithography) can define complicated patterns with nanometer resolution. However, since these techniques are typically restricted to working at a material interface or surface layer (that is, in a typically two-dimensional format), much effort is required to use these methods to define a pattern in three dimensions. In particular, many layers of such a two-dimensional patterned material must typically be united to create a three-dimensional patterned material. Many steps are required to produce each layer, and it therefore becomes prohibitive, both in terms of cost and time, to use these techniques (which are referred to herein as standard lithographic approaches) to build multi-layered structures.
One particular application in which a method for producing materials with a three-dimensional pattern is useful is in photonic crystals. A review of the properties and applications of such materials can be found in an article by Joannopoulos et al. entitled “Photonic Crystals: Putting a New Twist on Light,” Nature, Vol. 386, pp. 143-149 (Mar. 13, 1997). Simply stated, a photonic crystal is a material with a periodic index of refraction. When the modulation of the index occurs on a length scale comparable to the wavelength of light, the material can modify the propagation of the photon through the material via diffraction. The extreme example is a photonic crystal which possesses a complete photonic band gap, a range of energies for which the photon cannot propagate in any direction inside the material. Many applications and photonic devices have been predicted for photonic crystal structures that have a complete photonic band gap. However, to obtain a material with a complete photonic band gap, the photonic crystal must: (1) be made from very specific materials (typically, but not limited to, high refractive index materials such as semiconductors) and (2) have the correct three-dimensional structure to open a complete photonic band gap. Since these criteria are difficult to satisfy, photonic band gap materials that are suitable for optical wavelengths have been extremely challenging to fabricate and only a few examples have been realized. One example was recently reported by Noda et al., “Full Three-Dimensional Photonic Bandgap Crystals at Near-Infrared Wavelengths,” Science, Vol. 289, pp. 604-606 (Jul. 28, 2000). This structure was obtained by using standard lithographic approaches.
However, to avoid the inherent difficulties of using standard lithographic approaches, as discussed above, many researchers have recently been exploring so-called self-assembly methods to provide a much simpler and less expensive route to photonic crystals and photonic band gap materials. A common approach that is well known in the art is to utilize sub-micron colloidal spheres (e.g. polymer or silica), which can be induced to spontaneously order on a face-centered cubic (fcc) lattice. In nature, this process leads to gemstone opals. In analogy, sub-micron spheres assembled in the laboratory are referred to as synthetic opals. Unfortunately, as prepared, synthetic opals are not particularly interesting photonic crystals. For example, silica has a relatively low refractive index (about 1.4). However, since the interstitial spaces between the spheres are empty, they can be filled with other materials. In this way, the opal can be used as a template. Subsequent removal of the template, either by etching or burning away the spheres, leads to so-called inverted opals. A variety of such structures, including carbon, metal oxides, polymers, metals, and semiconductors can now be prepared using this procedure. In general, inverted opals have been studied since, in principle, they can have the proper symmetry (fcc), volume fraction (about 20%), and refractive index contrast (>2.85), necessary to obtain a complete photonic band gap at visible or near visible wavelengths. Thus, this approach has been explored as a simple method to obtain complete optical photonic band gaps.
One way to satisfy the criteria for the structure to have a complete photonic band gap is to fill the opal with a high refractive index material. Various methods have been proposed to achieve this. Milstein et al. have described, in very general terms, methods for preparing photonic band gap materials in which the pores of a reticulated template are filled with a high index material. See U.S. Pat. Nos. 5,385,114, 5,651,818 and 5,688,318. The high index material is incorporated into the template either as a liquid or gas and then solidified. Such an approach has been described in more detail in a recent paper by Blanco et al., “Large-Scale Synthesis of a Silicon Photonic Crystal with a Complete Three-Dimensional Bandgap Near 1.5 Micrometers,” Nature, Vol. 405, pp. 437-440 (May 25, 2000), where chemical vapor deposition of disilane is used to fill the opal template with silicon. After deposition, chemical etching is used to remove the template and a structure, defined as a “silicon inverted opal”, is obtained. Since silicon has an index of refraction of 3.5 in the near infrared, these structures can, in principle, satisfy all the criteria for a complete photonic band gap at 1.3 or 1.5 micrometers, the main wavelengths for optical communications. Thus, such structures could be extremely useful for making inexpensive photonic crystal devices for applications in telecommunications.
However, because Blanco et al. use a large (millimeter- or centimeter-scale) opal template to make silicon photonic crystals, this approach leads to several difficulties in terms of device applications. Blanco et al. use the most common method for preparing the synthetic opal, sedimentation. In sedimentation, colloidal spheres are mixed in a solvent and allowed to slowly settle and self-assemble onto a flat substrate. The final sediment is then sintered to obtain a macroscopic (millimeter- or centimeter-scale) template. While the resulting structure is a highly ordered crystal of sub-micron spheres on a local scale, due to the macroscopic size of sedimented opals, they are polycrystalline. In other words, the macroscopic opal is comprised of a large collection of small crystalline domains of spheres, roughly 50-100 μm in diameter. The random orientation of these individual domains, and the potential for disordered regions of spheres between the domains, causes a significant and undesirable deterioration in the photonic properties of these materials. In particular, any disorder or polycrystallinity that is present in the original opal template is automatically transferred into the silicon inverted opal.
Another problem with the use of sedimented opals, is that the resulting silicon photonic crystal is difficult to integrate into current optoelectronic technology. For example, it is desirable to place a photonic crystal device directly on a semiconductor substrate, such as a wafer, to facilitate interactions with other more traditional electronic devices on the same substrate. Furthermore, by placing the photonic crystal on a semiconductor wafer it could be more easily adapted into current semiconductor device fabrication lines. However, the large size, irregular shape, and polycrystallinity of inverted opals made from sedimented opals makes this difficult to achieve.
Accordingly, a need exists for a simple method to make photonic crystals that are easily integratable into current semiconductor wafer technology. The present invention describes how to avoid the problems caused by sedimented opals and prepare thin, planar, photonic crystals that are fabricated directly on a semiconductor wafer. Since the photonic crystals can be made from high refractive index materials, the resulting structures can exhibit a complete photonic band gap. Our approach not only allows integration with current semiconductor technology, but it is simpler, less time-consuming, and more adaptable to large-scale production than the prior art. Furthermore, once the thin photonic crystal is prepared by the method of this invention, it is easily processed with a variety of standard techniques to obtain integrated optical circuits.
Instead of using a sedimented opal as a photonic crystal template, the present invention provides a new form of template, referred to as a “planar opal”. The formation of planar opals relies on capillary forces to uniformly deposit a specific number of layers (e.g. 25) of close-packed colloidal spheres onto a large area substrate. This method has been used previously to make two-dimensionally periodic monolayers of spheres, see Denkov et al., “Two-Dimensional Crystallization”, Nature, Vol. 361, p. 26 (1993) and U.S. Pat. No. 5,540,951. More recently, it has been extended to make three-dimensional opaline structures. Jiang et al., see “Template-Directed Preparation of Macroporous Polymers with Oriented and Crystalline Arrays of Voids”, Journal of the American Chemical Society, Vol. 121, pp. 11630-11637 (1999), showed how to place a glass substrate vertically in a solution of colloidal spheres. Slow evaporation of the solvent under the appropriate conditions leaves a deposit of three-dimensionally ordered spheres on a face-centered cubic lattice. The spheres are deposited directly on the template. While point defects remain, these structures have the potential of being single crystals. Thus, these planar opals are superior to sedimented opals in that they are not polycrystalline, they are of a well-defined thickness, and they have a known crystal orientation.
However, Jiang et al. clearly state that their method does not work for colloidal spheres with diameters larger than about 400 nm. Spheres this large, referred to herein as “large spheres,” quickly sediment before the solvent can evaporate. Thus, the planar opal does not form. Unfortunately, for many important applications large spheres as defined above are required. This is because the size of the sphere determines the lattice constant of the photonic crystal. The lattice constant, in turn, determines the optical wavelength of the photonic band gap. To place the photonic band gap at the technologically interesting wavelength of 1.5 micrometers, the opal must be made of spheres of approximately 850 nm. Further, to obtain a complete photonic band gap in silicon photonic crystals, the position of the band gap must be below the absorption edge of the semiconductor (about 1.1 micrometers). This requires spheres larger than 640 nm. Therefore, a need exists for a simple method to form planar opals from large spheres.